The present invention relates generally to power amplifiers and, more particularly, to circuits used for improving the linearity of such amplifiers.
A power amplifier is one of the most important components of a wireless communication system, as it typically makes up the final processing stage before a generated radio-frequency (RF) signal is transmitted over an air interface by an antenna at a base station or a mobile terminal. From this stems the first requirement of a power amplifier, which is to be capable of providing power gain.
Furthermore, the power amplifier must provide sufficient gain over the frequency band of interest. This requirement can be especially demanding in the case of wide band wireless systems such as spread spectrum and in particular CDMA (code division multiple access), where the frequency band of interest is on the order of several Megahertz or more.
In addition, the power amplifier is required to provide as xe2x80x9clinearxe2x80x9d a response as possible. That is, the signal at the amplifier output should ideally be a scaled and delayed replica of the input to the amplifier, regardless of the amplitude or instantaneous frequency of the input signal. The consequences of a non-linear response include the introduction of xe2x80x9cin-bandxe2x80x9d distortion which may affect the error rate performance of the data signals being transmitted in the frequency band of interest, as well as the introduction of xe2x80x9cout-of-bandxe2x80x9d distortion which may affect the error rate performance of the communications systems used by other licensees in neighbouring frequency bands.
Therefore, it is crucial for power amplifiers to amplify the input signal in a highly linear fashion. However, most amplifiers which are sufficiently powerful and are capable of operating in a desired frequency range often suffer from a non-linear response characteristic. That is, the gain of the amplifier is not constant at all input levels. Furthermore, the group delay also tends to vary across the frequency range of operation.
Thus, in order to improve the linearity of power amplifiers, those skilled in the art have turned to the development of various xe2x80x9clinearizationxe2x80x9d techniques, including feedforward, feedback and pre-distortion amplifier design. Owing to its simplicity and high level of performance, the feedforward linearization technique has found widespread application in wireless communications systems.
In a feedforward amplifier configuration, the output of a xe2x80x9cmainxe2x80x9d amplifier (i.e., the amplifier requiring linearization) is corrected by adding an inverted error signal to the output signal. This error signal is generated by subtracting an attenuated version of the amplifier output from a delayed version of the amplifier input (shifted by 180 degrees in phase with respect to the output signal), and amplifying the result by means of an xe2x80x9cerrorxe2x80x9d amplifier. Thus, the error signal is in fact an estimate of the negative of the distortion produced by the main amplifier. Hence, when the error signal is added to the output of the main amplifier, the distortion is cancelled, resulting in improved amplifier linearity.
However, when very strict linearity requirements are imposed over a wide band of frequencies, the just described feedforward linearization technique might still not provide adequate distortion cancellation. This is also true in environments where the various components of the amplifier configuration are subjected to variations due to temperature or aging.
To this end, it has been proposed to use an adaptive technique in order to improve the linearity of a feedforward amplifier configuration and to provide a more predictable level of performance over a wide range of environmental operating conditions. In many cases, it has been found that use of a xe2x80x9cpilot signalxe2x80x9d for fine tuning the cancellation process can lead to improved performance.
In particular, an externally generated pilot signal is injected at the input of the main amplifier and the presence of the pilot signal is monitored after cancellation with the error signal. If performance of the feedforward configuration is ideal, the pilot signal should disappear after the error signal has been added to the output of the main amplifier. However, if part of the pilot signal remains, this means that the cancellation provided by the error signal is not ideal. Thus, a feedback loop is provided in order to adjust the magnitude and phase of the error signal so as to minimize the power of the distortion of the main amplifier which also minimizes the pilot signal after addition of the error signal.
Unfortunately, the just described pilot-assisted technique has several significant disadvantages which can in fact lead to degraded performance. Firstly, one may consider the problem of where to place the pilot signal in terms of frequency. If the pilot signal is placed inside the band of interest, then the pilot signal will interfere with the RF signal being transmitted across the air interface. The converse is also true, whereby any RF signal content at the pilot signal frequencies can affect the pilot receiver sensitivity and its architectural complexity, hence adversely affecting the performance of the feedback loop responsible for adjusting the error signal.
On the other hand, if the pilot signal is located outside the frequency band containing the desired RF signal requiring amplification, behaviour of the entire amplifier configuration will be biased because linearity at the pilot frequencies does not necessarily lead to linear behaviour in the band of interest. Furthermore, if the feedback loop does not sufficiently remove the out-of-band pilot signal to meet regulatory guidelines, a further filtering stage must be provided, which is not only expensive but can introduce additional performance degradation of the power amplifier (e.g., lowering the output power) and thus may offset any improvement afforded by the feedback loop itself.
Moreover, placing the pilot at the edge of the transmitted signal bandwidth requires an algorithm that is capable of moving and repositioning the pilot in case the transmitted signal is re-set to a new frequency channel. This introduces an additional level of complexity, which is highly undesirable.
It is therefore apparent that there is a need for improving the linearity of an amplifier configuration without resorting to the injection of a pilot signal that must subsequently be removed.
The present invention provides simplified and improved linearization of an amplifier without the disadvantages associated with the prior art. The invention relies on the use of an embedded pilot signal; that is, the signal being amplified contains both an information component and a pilot component that shares the same frequency region as the information component.
Accordingly, the invention can be summarized broadly as a circuit for processing an information signal in which is embedded a pilot signal, where the circuit is connectable to a configuration used for applying distortion correction to the output of an amplifier fed by the information signal. The information containing the pilot signal can be a spread spectrum signal such as a CDMA signal or a frequency-division multiplexed signal, for example.
The circuit of the invention basically includes first and second demodulators which are used for demodulating the embedded pilot signal at first and second points which are respectively located before and after the application of distortion correction by the configuration. The invention also includes a controller which is connected to the first and second demodulators. The controller varies the amount of distortion correction applied by the configuration as a function of the pilot signals demodulated by the first and second demodulators.
The invention is applicable to all types of configurations, including feedforward, feedback, feedforward-feedback and pre-distortion. In the case of feedforward, the configuration may include first and second signal paths. The first signal path may have a delay element connected to a fixed-gain element, where the delay element is tapped at the input to the amplifier, and where the fixed-gain element is coupled to the output of the amplifier. The second signal path may have an attenuator connected to a phase shifter, where the attenuator is tapped at a point between the output of the amplifier and the point at which the output of the fixed-gain element is coupled to the output of the amplifier, and where the phase shifter is coupled to the first signal path at the output of the delay element.
When this type of feedforward configuration is employed, the controller could be located in the first signal path between the point at which the phase shifter is coupled to the first signal path and the input to the fixed-gain element. Also, the xe2x80x9cfirst pointxe2x80x9d could be located at the output of the attenuator and the xe2x80x9csecond pointxe2x80x9d could be located at the output of a second attenuator tapped after the output of the fixed-gain element is coupled to the output of the first delay element. Other arrangements are possible, some of which involve the inclusion of delay elements tapped from the output of the amplifier or from the output of the attenuator.
Advantageously, by comparing and attempting to match the pilot component present after distortion cancellation to the pilot component present before distortion cancellation, there results improved linear performance of the amplifier configuration as a whole. This is partly due to the fact that the pilot component occupies the same frequency region as the information component. Because of the improved degree of linearization, it is possible to rate the configuration for higher nominal output power. It may even be possible to extend the operating bandwidth of the amplifier configuration.
Also, by cancelling distortion in a band of interest in the manner of the present invention, there will be no additional contamination of the power spectrum. Advantageously, therefore, the incidence of extraneous signals appearing in frequency regions outside or inside the band of interest will be reduced.